The present disclosure relates to discharge lamps. It finds particular application with regard to high intensity quartz or ceramic discharge lamps that operate at elevated temperatures, and to the use therein of a non-oxidizing barrier layer on the foil and/or coil assembly of the lamp, and to devices exhibiting a metal and/or a nonmetal/semiconductor interface. However, it is to be appreciated that the present disclosure will have wide application throughout the lighting and related industries.
Generally known in the art are fluorescent and other discharge lamps. These lamps have a glass tube or light-transmissive envelope, usually with a circular cross-section. This envelope houses the lamp electrodes, a supporting fill gas, and various mercury components known in the art. The envelope may be hermetically sealed at both ends. The lamp electrodes are mounted in one or more bases, usually positioned at the ends of the envelope, these bases also functioning to seal the envelope. The electrodes coordinate to provide an arc discharge, and are formed of very small diameter wire, usually tungsten.
The electrode-pin assembly are by their physical structure and nature fragile. In addition, they are subject to additional stress during lamp manufacture and during lamp operation, particularly at elevated temperatures. Due to these factors, the electrode assembly of the lamp often includes a foil connector, which provides physical support for the fragile electrode and also functions to absorb some of the heat build-up of the electrode, thus reducing electrode operating temperature and consequently increasing lamp life.
As another means to provide support for the fragile electrode assembly, lamps often exhibit what is commonly referred to in the art as a “pinch portion” of the lamp. This portion of the lamp, located just next to the base at either end of the envelope, is characterized by a reduced diameter of the envelope or tube where the glass has been “pinched” close to the electrode. However, while this does increase the stability of the electrode assembly within the lamp structure, it also generates a potential problem with regard to the glass envelope coming into direct contact with the electrode wire, and compromising the performance thereof. One means of offsetting this problem has been to provide a coil around the electrode/foil assembly in the pinch area to serve as a barrier between the glass envelope and the electrode assembly.
In the lamp having the above general configuration, the coil/foil assembly is used to reduce stress caused by the thermal expansion mismatch between the quartz envelope and the metal electrode. Generally, the foil is a molybdenum foil, though other metals may be used. Even with the addition of the metal foil to the electrode assembly, the lamp continues to experience thermal expansion mismatch problems. For purposes of this disclosure, the term “foil” is meant to refer to any metal used to connect the electrode to the power source of the lamp, and unless otherwise noted, the terms “foil” and “foil/coil” or any combination thereof refer to the support assembly for the electrode(s).
In practice, the foil is welded to the electrode, for example a tungsten electrode, though other electrode materials may be used. The area of the weld is located in an area of the lamp known as the pinch, as is noted above, called this because the quartz envelope is actually “pinched” together as a means of supporting the foil/electrode connection. The weld may be further wrapped with a metal coil, or encased in a tube, to ensure that the quartz does not directly contact the electrode, and to stabilize the area of the foil/electrode weld.
This foil/coil assembly, even though it is intended to provide support for the electrode, may fail for several reasons. For example, the assembly may fail due to structural weakness, or to continued thermal mismatch of the assembly component materials. In some instances, failure of the assembly is avoided by coating the foil with chrome to reduce oxidation of the metal during lamp operation. Another means used to reduce failure due to excessive heat build-up is the use of a sandblasting technique to frost the envelope, which increases surface area and reduces pinch temperature. This practice is often used in entertainment lighting fixtures because this type of fixture runs the lamp base temperature to more than 400° C.
Regardless of the intended use of a lamp, most lamps include at least the foil/coil assembly in the noted pinch portion of the lamp. Because quartz, used to make many lamp envelopes, is an oxide material and the foil is generally formed from a metal, for example molybdenum, there remains an inherent thermal mismatch between the quartz and the foil, which is emphasized in the pinch area where the two materials are in very close proximity. This correlates to a stress point in the structure, which under normal or conventional operating conditions will eventually lead to lamp failure, but which is amplified under high operating temperature conditions resulting in early lamp failure. For example, under high temperature operating conditions, the pinch portion of the lamp may experience temperatures in excess of 500° C. Because the molybdenum foil oxidizes at around 500° C., within the first 400 hours of operation the foil suffers oxidation-generated degradation which leads to early lamp failure. Therefore, use of the conventional foil/coil electrode support in extended or long operating lamps, on the average of up to 1000 to 2000 operating hours, is not a viable option.
It will be appreciated by one skilled in the art, given the foregoing, that there remains a void in lamp technology with regard to high operating temperature lamps which do not suffer from early lamp failure, or shortened operating life. The problem to be addressed requires a solution which will not only eliminate the thermal mismatch between the metal foil and the quartz envelope in the pinch area, but which will also function to avoid early oxidation of the foil at high operating temperatures, thus allowing long lasting lamp performance, in excess of 1000 hours, even though the operating temperature of the lamp is in excess of 500° C.